1. Field of the Invention
The present invention relates to a position control method for a shape memory alloy actuator.
2. Description of the Related Art
Conventionally, actuators using a shape memory alloy are controlled by a feedback control based on the shape memory alloy's own value of resistance that changes with deformation of the shape memory alloy. For example in the case of a shape memory alloy wire, changes in the resistance of the shape memory alloy with its deformation substantially conform to changes in the wire length. In view of this, the position (or length) is indirectly controlled by a feedback control based on the value of resistance of the shape memory alloy wire.
A feedback control based on the value of resistance of a shape memory alloy is disclosed, for example, in Japanese Patent Application Laid-Open No. 7-274561. FIG. 7 illustrates a conventional resistance-based feedback control system. A shape memory alloy actuator 101 is adapted to drive a load utilizing shape recovery of a shape memory alloy caused by a change in the temperature. The actuator 101 is provided with a PWM drive signal generator 103 that serves to change (or raise) the temperature of the shape memory alloy. The shape memory alloy is heated by power supplied by a PWM drive signal output from the PWM drive signal generator 103, whereby displacement of the shape memory is controlled. The PWM drive signal generator 103 receives a signal to be supplied to the shape memory alloy actuator 101 from a controller 105.
The controller 105 is provided with a PID (Proportional Integration Differential) controller 108. The PID controller 108 receives a resistance command value and the DC resistance component of the impedance of the shape memory alloy actuator 101 and outputs a signal to be supplied to the shape memory alloy actuator 101. A resistance detector 104 detects or measures the DC resistance component of the impedance of the shape memory alloy actuator 101 and outputs it to the PID controller 108 in the controller 105.
The resistance detector 104 is configured to constitute a bridge circuit in cooperation with the shape memory alloy actuator 101 so as to operate at a time when output of the PWM drive signal is off, that is, when no PWN drive signal pulse is output to the shape memory alloy actuator 101. The PID controller 108 receives a resistance command value supplied from the outside and the DC resistance component of the impedance detected by the resistance detector 104, and outputs a signal to the PWM drive signal generator 103.
The resistance command value is indicative of a commanded resistance value corresponding to the displacement amount of the shape memory alloy actuator 101. The controller 105 determines and controls the displacement of the shape memory alloy actuator 101 in accordance with the commanded resistance value input thereto.
The relationship between the temperature of a shape memory alloy that is heated by power supply and a resultant displacement thereof generally has a hysteresis. An actuator having such a hysteresis is a non-linear control objects as represented by the hysteresis. Prior art control apparatuses for an actuator have applied a linear control method such as PID feedback to such a non-linear control object. Under the present situation, there are no general methods of determining a PID gain with which satisfactory control is achieved all over the control range.
For example, in the case of a method in which control is performed with a constant gain across the board, control can be satisfactorily performed for some control target values, but oscillation may occur for some other control target values. Thus, the prior arts methods cannot improve the positioning accuracy beyond a certain limit.